In recent years, Ultra Super Critical Boilers of high efficiency, with enhanced steam temperature and pressure, have been built in the world. Specifically, to increase a steam temperature, which was about 600° C., to 650° C. or more or further to 700° C. or more, has been planned. Energy saving, efficient use of resources and reduction in the CO2 emission for environmental protection are the objects for solving energy problems, which are based on important industrial policies. And the high efficient Ultra Super Critical Boiler and furnace are advantageous for a boiler for power generation and a furnace for chemical industry, which burn fossil fuel.
High temperature and high pressure steam increases the temperature of a superheater tube for a boiler and a furnace tube for chemical industry, and a thick plate material and a forging, which are used as a heat resistant pressure member, and the like, during the actual operation, to 700° C. or more. Therefore, not only high temperature strength and high temperature corrosion resistance, but also excellent stability of a microstructure for a long period of time, excellent creep rupture ductility and excellent creep fatigue strength are required for the material used in such a severe environment for a long period of time.
Further, in the case of maintenance operations such as repairs after a long period of use, a material deteriorated by aging in a long period of time needs to be cut, worked, or welded, and therefore, in recent years, not only the characteristics for a new material but also the soundness for an aged material have been strongly required. In addition, from a viewpoint of practical use, the improvement in hot workability for the material used in the said severe environment has also been strongly required.
With regard to the above-described severe requirements, an Fe-base alloy such as an austenitic stainless steel suffers lack of creep rupture strength. Therefore, it is inevitable to use a Ni-base alloy in which the precipitation of a Y′ phase or the like is utilized.
Thus, the Patent Documents 1 to 8 disclose Ni-base alloys that contain Mo and/or W in order to achieve solid solution strengthening, and also contain Al and Ti in order to utilize precipitation strengthening of the Y′ phase, which is an intermetallic compounds and the specific formation thereof is Ni3(Al, Ti), for use in such a severe high temperature environment mentioned above. Furthermore, the alloys disclosed in the Patent Documents 4 to 6 contain 28% or more of Cr; and therefore a large amount of α-Cr phases having a bcc structure precipitate in the said alloys.